Copending application for Pat. Ser. No. 527,603 filed Nov. 24, 1974 for SERIAL IMPACT CALCULATOR PRINTER, now U.S. Pat. No. 3,986,594 and assigned to the assignee of the present invention discloses apparatus in which a matrix print head of the type including a compact arrangement of solenoid-actuated print wires can be reversibly driven transversely of the direction of movement of the recording medium at a closely controlled rate by a continuous loop, helical drive surface on a cylindrical roller which is rotatably driven by a synchronous motor drive at one end of the roller. Broadly, the configuration of the helical drive which is formed in the cylindrical surface of the roller will determine the rate of speed of the print head in advancing from a start position across the width of the recording medium for the printing operation followed by reverse movement in the return direction back to the starting point for the next line of print. In the preferred form disclosed in the hereinbefore referred to U.S. Pat. No. 3,986,594, the loop extends between diametrically opposed locations on opposite sides of a full cylindrical roller and the surface of revolution formed by the loop is such that when rotated it will establish a dwell portion to permit the print head carrier to undergo controlled acceleration to a constant velocity for printing; and at the end of the print line will cause the print head to rapidly decelerate as it undergoes a reversal in direction then to accelerate in the return direction back to its home or starting position.